An automated hematological analyzer is an apparatus in which the analysis of blood samples is automated. As an item measured by this automated hematological analyzer, there is MCV (mean corpuscular volume) of a blood sample. The MCV is obtained as the hematocrit divided by the red blood cell (also referred to thereinafter as RBC) count. The hematocrit is the percentage of red blood cells in unit volume of whole blood. Accordingly, MCV is a value indicative of the mean volume of red blood cell. MCV is expressed in unit femtoliter (fL) or 10−15 L.
A sheath flow DC detection method is known as a method of determining RBC count and hematocrit by an automated hematological analyzer. In the sheath flow DC detection method, the number and size of blood cells are measured by detecting a change in impedance generated upon passage of blood cells through a fine pore arranged in a flow cell. When a blood sample is analyzed with this automated hematological analyzer, the blood sample should be diluted with a physiologically isotonic diluent. As general diluents for diluting whole blood, there are physiological saline, Ringer solution, Rocke solution, and Tyrode solution.
A blood sample-diluting reagent described in U.S. Pat. No. 6,225,124 is known as a blood sample-diluting reagent for preventing a change with time in the MCV of a blood sample. The blood sample-diluting reagent described in U.S. Pat. No. 6,225,124 contains at least one nonionic surfactant and a substance for regulating the osmotic pressure of the reagent in the range of about 150 to 400 mOsm/kg.
However, when the blood sample-diluting reagent described in U.S. Pat. No. 6,225,124 is used to measure the MCV of a blood sample, the MCV is changed depending on measurement temperature. More specifically, the MCV is significantly reduced as the temperature increases from low temperature. Then, the MCV has a flexion point at a certain temperature. Then, when the temperature at the flexion point is exceeded, the MCV is gradually increases as the temperature increases.
Generally in measurement of the MCV of a blood sample with an automated hematological analyzer, the measurement temperature is often 20° C. or more. However, the measurement temperature may become less than 20° C., depending on the environment under which the automated hematological analyzer is installed or on the performance of a heater in the automated hematological analyzer. Accordingly, when the flexion point is 20° C. or more, the correction, with measurement temperature, of measurement results from the automated hematological analyzer requires complicated correction. As a result, in measurement of MCV of blood samples with the automated hematological analyzer, there has been a problem that errors in measurement results are caused by measurement temperature.